A backlight device arranged in a conventional keyboard is usually for a dim environment or special use requirements (such as appearance). An illuminating keyboard is provided with light-emitting diodes at respective specific positions below the respective keys so that the keys can be penetrated by light. A common solution for uniformizing the light source is to disposing the light emitting diodes at a side of a light guiding sheet, and light is project into the light guiding sheet from the light emitting diode emits and diffused in the light guiding sheet.
The aforementioned configuration generally has a substrate, light emitting diodes (LEDs) are disposed on one side of a top surface of the substrate, control circuits electrically connected to the light emitting diodes are disposed on the top surface of the substrate, and a light guiding sheet is further disposed on the top surface of the substrate. Edge light LEDs are used in the aforementioned configuration, and the edge light LEDs illuminate long a direction parallel with the substrate and therefore can directly project light into the light guiding sheet. However, it is limited by current technologies, a thickness of the edge light LED is larger than even times of a thickness of a conventional direct light LED (the direct light LED projects light along a direction perpendicular to the substrate). Therefore, a total thickness of the aforementioned structure is limited by the thickness of the edge light LED and therefore difficult to be reduced. A conventional mobile device lacks of thickness space. If the thickness of the keyboard cannot be reduced, large electronic components are therefore forced to be gathered and disposed outside of the keyboard. However, the most areas in the mobile device are occupied by the keyboard, and an arrangement of the electronic components in the mobile device is seriously limited by space in the mobile device.
In views of this, in order to solve the above disadvantage, the present inventor studied related technology and provided a reasonable and effective solution in the present disclosure.